The Stability of Icosahedral Cd-Yb
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The Stability of Icosahedral Cd-Yb Günter Krauss1, Sofia Deloudi1, Andrea Steiner1, Walter Steurer1, Amy R. Ross2, Thomas A. Lograsso2 1 Laboratory of Crystallography, Department of Materials, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland 2 Ames Laboratory, Ames, IA, USA ABSTRACT The stability of single-crystalline icosahedral Cd-Yb was investigated using X-ray diffraction methods in the temperature range 20 K ≤ T ≤ 900 K at ambient pressure and from ambient temperature to 873 K at about 9 GPa. Single-crystals remain stable at low temperatures and in the investigated HP-HT-regime. At high temperatures and ambient pressure, the quasicrystal decomposes. The application of mechanical stress at low temperatures yields to the same decomposition, the formation of Cd. A reaction of icosahedral Cd-Yb with traces of oxygen or water causing the decomposition seems reasonable, but a low-temperature instability of this binary quasicrystal cannot be ruled out totally. INTRODUCTION What stabilizes quasicrystals? Are they stabilized by energy or entropy? Energy stabilization would mean that the quasicrystalline state is a ground state of matter. In case of entropy stabilization, quasicrystals would only be stable in a high temperature regime. The recently discovered congruently melting binary icosahedral Cd5.7Yb [1] could be very valuable to answer this question. As icosahedral Cd5.7Yb and its neighboring approximant phases Cd6Yb and Cd3.6Yb are line phases [2], chemical disorder can be excluded as essential entropic contribution. Therefore, the stability of icosahedral Cd5.7Yb at non-ambient conditions can give valuable hints to answer the question of stabilization. The corresponding icosahedral Cd-Ca was found to be stable at ambient conditions [3]. At 110 K, an order-disorder transition was reported for the cubic Cd6Yb approximant [4], which is also visible as anomalies in electrical resistivity and thermal conductivity, but was not found for the icosahedral Cd-Yb [5]. At high pressures, icosahedral Cd-Yb seems to remain stable up to 40 GPa [6].
EXPERIMENTAL DETAILS Icosahedral Cd-Yb was synthesized from appropriate quantities of Cd and Yb (4N) in a sealed Ta-crucible with a low partial pressure of ultrahigh purity Ar. The alloy was homogenized three times at 670 °C for one hour with a 180 ° rotation of the crucible between each step. Single crystals were grown by the use of the Bridgman technique. The homogenized alloy was heated to 800 °C and after one hour the crystal was grown with a pulling speed of 1 mm/hr. A singlecrystalline sample was oriented and sectioned from the ingot. After dividing it into smaller grains, selected single crystals were stored in a glove-box under Argon atmosphere. X-ray diffraction experiments were carried out on a mar300 image-plate system with Mo-Kα radiation (Johansson monochromator, RAG 50 kV, 80 mA). For Laue experiments the monochromator
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was removed. Powder diffraction experiments were done on a Stoe Stadi-P powder diffractometer using Cu-Kα radiati
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